A youtuber called Of Herbs And Alters makes videos about their own recurring anorexia battle and has done some deep dives into why *some* people 'choose' anorexia. Clearly some of this does align with Eugenia, one obvious point being she is not hiding away from the world in baggy clothing for example, quite the opposite. It's very complicated I know, there's a lot going on with her and it's certainly not going to get better anytime soon.
As someone who has gone through a long battle with AN I personally do not believe someone can just BECOME sick. Just like you can not choose to have depression, you also can not choose to suddenly wake up and have AN. That is just what I believe. Someone else can have different experiences.
I didn't start hiding my body until people made me feel uncomfortable and ashamed (my bar is very low), not because I felt uneasy myself. When people tell Eugenia she looks beautiful, there is no need for her to feel ashamed and because she's been online very long, she's more resiliant.
I honestly don't believe you can actively decide to have AN. You can decide not to find treatment, but I firmly believe that this is your illness speaking and taking over because that damn monster wants to win.